1. Field of the Invention
The present invention relates to an epitaxial (hereinafter referred to as xe2x80x9cEpixe2x80x9d) structure for low ohmic contact resistance and a method for growing such a structure.
2. Description of the Prior Art
In recent, GaN-based semiconductor devices have been put into the spotlight all over the world. GaN-based semiconductors are characteristic of large energy band gaps (Eg=3.4 eV) in comparison with conventional group III-V compound semiconductors. Numerous applications exist in the optical devices and electronic devices which stand in need of such characteristics.
The GaN-based optical devices are exemplified by light emitting diodes and laser diodes, which emit blue light, found in the short wavelength side of the visible light spectrum, and are applied for displays which require the three primary colors and high density information storage devices such as CD pickup, etc.
Applications of the GaN-based electronic devices can be found in field effect transistors (FET) and hetrojunction bipolar transistors (HBT), which take advantage of the high speed operation possibility due to excellent electron mobility and the high temperature and high power operation possibility due to large energy band gaps. To date, active research has been directed to the development of the GaN-based electronic devices which require high temperature and high power operation.
Generally, preparing techniques of GaN-based optical or electronic devices find difficulty in forming low p-type ohmic resistance owing to the following three reasons:
First, the band gap of GaN as large as 3.4 eV induces a large potential barrier ("PHgr"b1) when the metal used for ohmic formation is brought into contact with p-type GaN, resulting in the formation of a very large ohmic resistance (the current flowing through an ohmic contact is proportional to an exponential function of the potential barrier, exp (xe2x88x92"PHgr"b).
Next, the potential barrier ("PHgr"b1) which Ni, a most typically used p-type ohmic metal of GaN, forms along with GaN, is very large, identified as amounting to two thirds (about 2.25 eV) of the band gap of GaN, as shown in FIG. 4c. 
Finally, the p-type impurity density of GaN is generally smaller than the n-type impurity density and the tunneling effective mass of holes is larger than that of electrons, so a relatively large resistance is formed at p-type GaN upon ohmic contact.
In association with such a large resistance, there occur problems in the preparing techniques of GaN-based optical or electronic devices.
First, optical devices, as shown in FIG. 4d, demand higher voltages to generate the same amounts of an optical signal (proportional to current). This leads to decreasing the luminescence efficiency of optical devices and increasing their power consumption. Further, the large amount of heat generated owing to large ohmic resistance may decrease the reliability of optical devices.
In the case of the electronic devices using p-type ohmic electrodes, such as HBT, large ohmic resistance acts decisively to deteriorate their velocity characters.
With reference to FIG. 4, there is illustrated a conventional technique of forming an ohmic electrode of a p-type GaN-based semiconductor. As shown in FIG. 4a, a conventional Epi structure has an ohmic metal 2, such as nickel, deposited on a p-type GaN 1 atop an n-type GaN 3 which is formed above a substrate 1 with a buffer 2 being interposed therebetween. The ohmic metal 2 is subjected to thermal treatment to increase the p-type impurity density of GaN at the ohmic contact surface. In FIGS. 4b and 4c, reference letter Eg denotes an energy band gap in which no free electron xc3xaa- and hole ĥ-allowable energy levels exist. Reference letters EFM and ESM stand for Fermi levels and VON and ION denote the turn on voltage and current of diodes, respectively.
The increasing of the p-type impurity density of GaN brings about reducing the width of the potential barrier which takes place in the ohmic contact, thereby increasing the tunneling current component to effect the attenuation of ohmic resistance. However, since the height of the potential barrier is not greatly affected, as low p-type ohmic resistance as desired is rarely achieved.
In conventional techniques, the density of p-type GaN at ohmic contacts is increased through thermal treatment, so as to reduce the width of the potential barrier formed. The reduction in the width of the potential barrier increases the tunneling probability of holes through the potential barrier, resulting in an increase in the current which flows through the ohmic resistance (that is, the resistance is reduced). However, the reduction effect of resistance is limited owing to the fact that the height of the potential barrier is not affected so that the flow of a number of holes is still blocked by the high potential barrier.
Therefore, the present invention has a technical object of lowering the potential barrier occurring in a p-type GaN to significantly reduce the ohmic resistance thereof, allowing the provision of high performance GaN-based optical devices or electronic devices.
In an aspect of the present invention, there is provided an Epi structure for low ohmic contact resistance in p-type GaN-based semiconductors, comprising a p-type GaAs which is doped at a very high density between an ohmic contact metal and a p-type GaN and subjected to crystal growth, wherein a potential barrier formed in the p-type GaN can be reduced upon the formation of ohmic contact on the p-type GaN.
In another aspect of the present invention, there is provided an Epi structure for low ohmic contact resistance in p-type GaN-based semiconductors, comprising a p-type AlxGa1xe2x88x92xAs (0 less than xxe2x89xa61) which is doped at a very high density and graded between an ohmic contact metal and a p-type GaN and subjected to crystal growth, wherein a potential barrier formed in the p-type GaN can be reduced upon the formation of ohmic contact on the p-type GaN.
In accordance with the present invention, the Epi structure is characterized in that the ohmic contact metal is a non-alloyed ohmic contact metal.
In accordance with the present invention, the Epi structure is characterized in that the ohmic contact metal is an alloyed ohmic contact metal and thermally treated to increase the p-type impurity density of the p-type GaN, wherein the width of the potential barrier formed between the doped GaAs or graded AlxGa1xe2x88x92xAs (0 less than xxe2x89xa61) and the p-type GaN can be reduced.
In a further aspect of the present invention, there is provided a method for growing an Epi structure for low ohmic resistance in p-type GaN-based semiconductors, comprising the steps of: depositing a p-type GaAs at a very high density onto a p-type GaN; forming a non-alloyed ohmic metal layer over the p-type GaAs; and subjecting the p-type GaAs to crystal growth, whereby a potential barrier formed in the p-type GaN can be reduced upon the formation of ohmic contact on the p-type GaN.
In still a further aspect of the present invention, there is provided a method for growing an epitaxial structure for low ohmic resistance in p-type GaN-based semiconductors, comprising the steps of: depositing a p-type AlxGa1xe2x88x92xAs (0 less than xxe2x89xa61) at a very high density onto a p-type GaN; forming an alloyed ohmic metal layer over the p-type GaAs; thermally treating the ohmic metal layer; and subjecting the p-type AlxGa1xe2x88x92xAs to crystal growth.